Navigating the translational roadblock: Towards highly specific and effective all-optical interrogations of neural circuits

Two-photon (2-P) all-optical approaches combine in vivo 2-P calcium imaging and 2-P optogenetic modulations and have the potential to build a framework for network-based therapies, e.g. for rebalancing maladaptive activity patterns in preclinical models of neurological disorders. Here, our goal was to tailor these approaches for this purpose: Firstly, we combined in vivo juxtacellular recordings and GCaMP6f-based 2-P calcium imaging in layer II/III of mouse visual cortex to tune our detection algorithm towards a 100 % specific identification of AP-related calcium transients. False-positive-free detection was achieved at a sensitivity of approximately 73 %. To further increase specificity, secondly, we minimized photostimulation artifacts as a potential source for false-positives by using extended-wavelength-spectrum laser sources for optogenetic stimulation of the excitatory opsin C1V1. We achieved artifact-free all-optical experiments performing photostimulations at 1100 nm or higher and simultaneous calcium imaging at 920 nm in mouse visual cortex in vivo. Thirdly, we determined the spectral range for maximizing efficacy of optogenetic control by performing 2-P photostimulations of individual neurons with wavelengths up to 1300 nm. The rate of evoked transients in GCaMP6f/C1V1-co-expressing cortical neurons peaked already at 1100 nm. By refining spike detection and defining 1100 nm as the optimal wavelength for artifact-free and effective stimulations of C1V1 in GCaMP-based all-optical interrogations, we increased the translational value of these approaches, e.g. for the use in preclinical applications of network-based therapies. One Sentence SummaryWe maximize translational relevance of 2-P all-optical physiology by increasing specificity, minimizing artifacts and optimizing stimulation efficacy.